1. Field of the Invention
This invention relates to a pilot signal detecting method, a pilot signal detecting apparatus, and a receiver, which are suitably applied to a mobile communication system which employs a code division multiple access scheme.
2. Description of the Related Art
In a code division multiple access (CDMA) scheme, a particular code (hereinafter this code is referred to as the pseudo random noise sequence (PN) code) assigned to each line is used to spread modulated waves at the same carrier frequency over a wider frequency band than its original frequency band (this technique is referred to as "spread-spectrum"), and respective spread-spectrum modulated waves are multiplexed for transmission. Received spread-spectrum signals are synchronized with a PN code applied through a line which carries a signal to be demodulated, so as to only identify a desired line.
More specifically, the transmission side of the system assigns a different PN code for each line. Here, the PN code comprises a series of pseudo random codes. Then, the transmission side multiplies respective modulated waves to be transmitted through the lines by the respective PN codes to spread the modulated waves. In this regard, the respective modulated waves undergo predetermined modulation processing prior to the spread spectrum processing. The respective modulated waves thus spread are multiplexed for transmission.
On the reception side of the system, on the other hand, received signals transmitted thereto from the transmission side are multiplied by the same PN code as that assigned to a line which carries a target signal to be demodulated, while maintaining the synchronization between the received signals and the PN code. In this way, only modulated waves transmitted through the target line are demodulated.
As described above, the CDMA scheme allows the transmission side and the reception side to directly communicate with each other every time a call is made, with the only condition being that the same PN code be set in both the transmission side and the reception side. Also, since the CDMA, scheme spreads modulated waves using different PN codes for respective lines, the reception side can demodulate only a spread spectrum signal transmitted thereto through a target line which carries the signal to be demodulated. In addition, since the PN codes include a series of pseudo random codes, it can be said that the CDMA scheme is excellent in security.
In a mobile communication system adopting the CDMA scheme, a base station on the transmission side repetitively transmits a PN code for mobile stations to acquire and maintain synchronization as well as to restore a clock therefrom (hereinafter, this PN code is referred to as the "pilot signal"). A mobile station on the reception side detects respective pilot signals transmitted by a plurality of base stations to assign each detected timing to a demodulator thereof. The mobile station generates the PN code in the demodulator. The demodulator multiplies a spread-spectrum signal transmitted thereto from a target base station by the PN code at an assigned timing to demodulate a desired signal from the target base station.
Stated another way, in a mobile communication system adopting the CDMA scheme, each base station transmits a PN code at different timings from each other. A mobile station detects the timing of the pilot signal supplied from a target base station and synchronizes the PN signal generated in its demodulator with the detected timing of the pilot signal, thereby making it possible to correctly demodulate a spread-spectrum signal transmitted from the target base station.
Note that, although the respective base stations transmit their own PN codes at timings different from each other as mentioned above, the PN codes themselves comprise the same code pattern. Stated another way, the difference in timing of the PN codes between the respective base stations exactly corresponds to the difference in their PN codes.
The pilot signal as mentioned above may suffer from a deviation in reception timing thereof due to movements of the mobile stations and changes in external environment through which the pilot signal is being received. The deviation in reception timing leads to a deviation in synchronization between the timing of a PN code assigned to a demodulator and the timing of the pilot signal. Generally, the deviation in timing is canceled by changing the timing of the PN code generated in the demodulator, thus keeping track of the deviated timing.
However, if fluctuations in timing due to the deviation are larger than the canceling capability of the demodulator, correct demodulation of a received signal is prevented by the deviated timing (so-called out-of-lock). In this case, the mobile station assigns a new timing to the demodulator in the out-of-lock state. In this event, the demodulator is assigned a timing of a pilot signal having a higher reception level as a new timing. Thus, it is highly possible to again assign a timing of a pilot signal which has been used in a demodulator that has experienced out-of-lock. If the same timing is again assigned to the demodulator, the assigned pilot signal can be captured while the assignment of the timing is repeated several times, provided that the mobile station is in a stand-by state.
However, as a reception level of the pilot signal becomes too low to be detected, the out-of-lock state may occur, for example, as is the case of a call made from a mobile station located near the boundary of the coverage of a base station. In such a case, even if the same timing of the pilot signal is again assigned to the demodulator, which has once experienced out-of-lock with this timing, the reception level itself remains reduced unless the external environment around the mobile station is largely improved. Therefore, a similar out-of-lock state is highly likely to occur again in the demodulator. As a result, the mobile station will fall into a so-called ping-pong condition in which the mobile station repeats processes of out-of-lock, detection of the pilot signal, and reassignment of the timing to the demodulator.
Consequently, the mobile station is burdened with an extremely large amount of pilot signal processing, thus resulting in a problem that the communication quality is degraded.
To avoid the problem mentioned above in order to maintain the communication quality during a call, it is necessary to immediately assign a timing of a pilot signal which exhibits a stable magnitude level, to a demodulator in the out-of-lock state, so that the demodulator can demodulate a received signal with the assigned timing.